1. Field of The Invention
The present invention relates to a method for producing grain oriented electrical steel sheet and strip whose grains have the orientation of {110} &lt;001&gt; and which is easily magnetizable in the rolling direction.
1. Description of The Prior Art
The magnetic properties required by grain oriented electrical steel are a high magnetic flux density and low iron loss characteristics.
Recently, increasing demands have been made for miniaturization and higher performance of electrical appliances, and for this purpose, it is necessary to reduce the weight of iron cores.
In general, in order to reduce the weight of iron cores used in various electrical applicances, a high degree of magnetic flux density must be utilized so that magnetic materials having good magnetization characteristics, namely good B.sub.8 characteristic (magnetic flux density at magnetization strength of 8 AT/cm) are required.
As compared with a magnetic material having a low B.sub.8 characteristic, a magnetic material having a high B.sub.8 characteristic shows much lower iron loss at a high magnetic field and shows a low increase in rate of the iron loss with an accompanying increase in the magnetic flux density.
In view of the above requirements, the improvement in the magnetic density which is naturally required with an increase in the size of electrical applicances will be realized only by the development of high magnetic flux density grain-oriented electrical steel sheets.
The present invention has as its object the supplying of products meeting such demands, and provides grain-oriented electrical steel sheets with a high magnetic flux density, exhibiting the excitation characteristic, i.e., B.sub.8 characteristic of at least 1.90 Wb/m.sup.2 in the rolling direction which is far superior to the conventional grain-oriented electrical steel sheet.
Regarding the production of a high magnetic flux density steel sheet, steel materials containing a small amount of acid soluble Al (herein called simply Al) are used, which are disclosed in Japanese Patent Publications Sho 33-4710, Sho 40-15644 and Sho 46-23820, which correspond to U.S. Pats. Nos. 3,159,511; 3,287,183 and 3,636,579, respectively. The feature of this prior art is that a strong reduction between 81 and 95% is applied in the final cold rolling step utilizing the effects of AlN.
Generally, in the production of grain-oriented electrical steel sheets, excellent magnetic characteristics in the rolling direction can be obtained due to the secondary recrystallization of Goss structure showing an orientation { 110 } &lt;100 &gt; in the final annealing, and in this case precipitates, such as, nitrides, sulfides and oxides formed by addition elements pay an important role. Conventionally, the contribution of these precipitates has been considered to restrict the grain growth of the matrix by their finely dispersed precipitation into the matrix and to promote the secondary recrystallization.
However, in the case of AlN, as disclosed in U.S. Pat. No. 3,626,309, for example, AlN precipitates in a specific orientation in relation to the matrix and has the ability to make the grains of a specific orientation grow selectively, and strictly controls the orientation of the secondary recrystallization grains so that products having excellent B.sub.8 characteristics can be obtained.
The present inventors have found that the diameter of the rolls used in the above strong reduction cold rolling have a great influence on the magnetic properties of the final products in the production of high magnetic flux density electrical steel sheets utilizing the effects of AlN.
It is already well known that a small-diameter roll can be used for rolling common steels. As an example of this small-diameter roll may be cited the Senzimir mill described in the Encyclopedia of the Iron and Steel Industry by A. K. Osborne (New York, 1956). An attempt to manufacture thin (0.5 - 7 mils) silicon steel sheets by using a Senzimir mill, is disclosed by M. F. Littmann in the U.S. Pat. No. 2,473,156 specification. The highest magnetic flux density obtained in the silicon steel sheet proposed by M. F. Littmann is .mu..sub.H.sub.=10 = 1815 (example D) at the highest. Moreover, the starting material and cold rolling reduction ratio must be strictly controlled in order to obtain such magnetic flux density. However, it can not be said that the grain-oriented electrical steel sheet possesses high characteristic value in case .mu..sub.H.sub.=10 is about 1815. No comment was made on the inhibitor effective for the growth of { 100 } &lt;100&gt; structure at the time of secondary recrystallization.